core/types: remove LogForStorage type (#23173)

The encoding of Log and LogForStorage is exactly the same
now. After tracking it down it seems like #17106 changed the
storage schema of logs to be the same as the consensus
encoding.

Support for the legacy format was dropped in #22852 and if
I'm not wrong there's no reason anymore to have these two
equivalent types.

Since the RLP encoding simply contains the first three fields
of Log, we can also avoid creating a temporary struct for
encoding/decoding, and use the rlp:"-" tag in Log instead.

Note: this is an API change in core/types. We decided it's OK
to make this change because LogForStorage is an implementation
detail of go-ethereum and the type has zero uses outside of
package core/types.

Co-authored-by: Felix Lange <fjl@twurst.com>